1. Field of the Invention
The present invention relates to thin-film coils, coil driving circuits and magnetic heads which are used for recording data in a magnetic recording medium.
2. Description of the Related Art
Recording of data on magnetic recording media is performed by controlling the direction of the magnetic flux of a recording magnetic field generated by a thin-film coil provided in a magnetic head. The direction of magnetic flux of the recording magnetic field with respect to the magnetic recording medium is alternated rapidly and continuously by driving the thin-film coil. In applying the recording magnetic field to the recording medium, the alternation of the direction of the magnetic flux should be made as quickly as possible. For this reason, the magnetic head is provided with a spiral coil which has a two-layer structure (See JP-A-2003-51144 for example).
Conventionally, the magnetic head for recording data in such away as the above has, for example, the construction shown in FIG. 13. A magnetic head H′ includes a transparent substrate 80, an object lens 81 attached to the transparent substrate 80, a thin-film coil 82 and a magnetic film 83. The thin-film coil 82 generates a recording magnetic field, and includes spiral coils 82a, 82b which are formed in two layers. These spiral coils 82a, 82b are coated with a dielectric layer 86 for insulation. In each layers of the spiral coils 82a, 82b, their respective outer circumferential ends 82aa, 82ba are insulated from each other and are connected to lead wires 85a, 85b respectively, which extend radially of the thin-film coil 82. Further, these lead wires 85a, 85b are connected to wiring members 86a, 86b which extend out of the transparent substrate 80. Though not illustrated in FIG. 13, each of the spiral coils 82a, 82b in the respective layers has an inner circumferential end which is connected to a common terminal that provides a predetermined reference potential. In these spiral coils 82a, 82b, wires are wound in the opposite directions to each other. The magnetic film 83 is provided by a material which has a high magnetic permeability, and is formed below the thin-film coil 82.
According to such a construction, an electromagnetic operation as shown in FIG. 14 for example is achieved. Specifically, with the reference potential at the common terminal being −V, one spiral coil 82a generates a magnetic field which has an upwardly-oriented magnetic flux when a voltage +V is applied to the outer circumferential terminal 82aa. On the other hand, the other spiral coil 82b generates a magnetic field which has a reverse or downwardly-oriented magnetic flux when the same voltage +V is applied to the outer circumferential terminal 82ba. In other words, the coil driving circuit for driving the thin-film coil 82 switches the states of energization of the spiral coils 82a, 82b exclusively to each other and at a high speed, based on a high-frequency signal which represents data to be recorded. In this manner, the recording magnetic field works on a magnetic recording medium D while the direction of magnetic flux is alternated at high speed, and data is recorded upon formation of magnetic domains which are given a polarity corresponding to the direction of the magnetic flux.
However, according the conventional technique described above, one spiral coil 82a is in a layer which is closer to the magnetic recording medium D while the other spiral coil 82b is in a layer which is farther, sandwiching the spiral coil 82a. With this construction, if the number of windings and the size of the coil are the same in both of the spiral coils 82a, 82b, the recording magnetic field generated by the spiral coil 82b which is in the layer farther from the magnetic medium D tends to be weaker than the recording magnetic field generated by the spiral coil 82a which is in the layer closer to the medium, which results in instability in the intensity of the recording magnetic field.
As a solution to such a problem, the number of windings and the size of the spiral coils 82a, 82b in respective layers could be different from each other, thereby stabilizing the intensity of the recording magnetic field. This is not preferable, however, because the spiral coils 82a, 82b then have different inductance values from each other, generating recording magnetic fields whose waveforms are different from each other depending upon whether the magnetic flux is pointing upward or downward. Even if the spiral coils 82a, 82b are constructed so as to have the same inductance, there is still a need for adjusting electric current values in the coil driving circuit when each of the spiral coils 82a, 82b are energized, which results in another problem of complex construction of the coil driving circuit.